Etic coils of corpusular beam apparatus control and regulating arrangement for the current fed to electromagn

ABSTRACT

An improved setting device for providing accurately reproducible, incrementally variable control voltages to the input of a control and regulating circuit for adjusting electromagnetic coil current in charged-particle beam apparatus such as electron microscopes. The setting device includes a digital counter, preferably a forward-backward counter, connected to the input of the control circuit through a digital-to-analog converter. A pulse source, preferably pushbutton-controlled, provides stepping pulses, either one-by-one, or in a series of pulses of variable repetition rate and/or preselectable number, for adjusting the output voltage of the setting device to achieve, for example, desired values of magnification or focus as indicated by a numeric display device connected to the output of the pulse source and/or the output of the digital counter.

United States Patent Miiller et al. Sept. 4, 1973 54] CONTROL AND REGULATING 3,715,582 2/1973 Akahori et al 250/495 A ARRANGEMENT FOR THE CURRENT FED TO ELECTROMAGNETIC COILS OF CORPUSCULAR BEAM APPARATUS Inventors: Karl-Heinz Miiller; Volker Rindfleisch, both of Berlin, Germany Assignee: Siemens Aktiengesellschaft, Munich,

Germany Filed: Sept. 15, 1972 Appl. No.: 289,532

Foreign Application Priority Data References Cited UNITED STATES PATENTS 8/1971 Gutter 250/495 A Primary Examiner-James W. Lawrence Assistant Examiner-B. C. Anderson Attorney Hugh A. Chapin, Richard L. Mayer et al.

[57] ABSTRACT An improved setting device for providing accurately reproducible, incrementally variable control voltages to the input of a control and regulating circuit for adjusting electromagnetic coil current in charged-particle beam apparatus such as electron microscopes. The setting device includes a digital counter, preferably a forward-backward counter, connected to the input of the control circuit through a digital-to-analog converter. A pulse source, preferably pushbutton-controlled, provides stepping pulses, either one-by one, or in a series of pulses of variable repetition rate and/or preselectable number, for adjusting the output voltage of the setting device to achieve, for example, desired values of magnification or focus as indicated by a numeric display device connected to the output of the pulse source and/or the output of the digital counter.

12 Claims, 1 Drawing Figure CONTROL AND REGULATING ARRANGEMENT FOR THE CURRENT FED T ELECTROMAGNETIC COILS OF CORPUSCULAR BEAM APPARATUS BACKGROUND OF THE INVENTION This invention relates to systems for controlling electric current, and more particularly to systems for stepwise adjustment of current flowing through electromagnetic coils for influencing beams of charged particles.

There is a large class of apparatus, including electron microscopes, ion microscopes, X-ray microscopes, diffraction equipment, devices for machining by streams of charged particles, cathode-ray oscilloscopes, and so forth, in which the magnetic fields of electromagnetic coils are used to influence beams of charged particles.

With particular reference to electron microscopes, several electromagnetic coils, forming so-called electromagnetic lenses, may be used to provide the desired magnification and focusing. In a typical electron microscope, for example, there may be a condenser lens, a diffraction lens, an intermediate lens, and an objective lens. The first three lenses primarily influence the magnification of the microscope, while the objective lens controls the focus.

Because it is usually necessary to operate the micro scope over a range of magnification values and focus lengths, depending on the specimen being investigated, each electromagnetic lens is provided with a control circuit for adjusting the coil current. A conventional setting device for providing variable control voltages to the input of such a control consists of merely a potentiometer, either continuously variable or preferably variable in steps by means of a multi-position selector switch.

Although electron microscopes normally have an image screen to permit direct visual examination of the generated image, in most applications a clearer image is obtained by making a photographic recording. This is particularly true where high resolution is desired. To ensure the sharpest possible photograph, the usual procedure is to set the objective lens control to a predetermined assumed best focus value for the chosen magnification and then to vary the lens current in a series of accurately determined steps about the present value.

For each magnification value there will be a different focus value, but it has been found in practice that a relatively small number of magnification steps is sufficient for most purposes. It is important, however, that the corresponding magnification values be reproducible as accurately as possible. 1

In addition to the requirement for accurately repeatable stepwise adjustment of the respective lens currents in the electron microscope, it is also necessary to accurately stabilize the lens currents during photographic exposure periods as long as several minutes. For example, in many cases the absolute value of current fluctuations must be limited to within 10 to of the total current. Thus, it is customary also to provide regulating circuits for each lens coil in the electron microscope.

A proposed circuit that combines the above described control and regulating functions includes a power amplifier with its input connected to the output of a differential amplifier having two inputs, one for a control voltage and the other for a stable reference voltage. The output of the power amplifier is connected in series with the electromagnetic lens coil and a measuring resistor across current source. Current regulation is obtained by inverse feedback from the measuring resistor to the control input of the differential amplifier Control voltages related to different coil currents are obtained from a setting device, such as the above-mentioned multi-position potentiometer, which is also connected to the input of the differential amplifier.

Because of the need for accurate repeatability and current stability, the resistors in the setting device must be precision resistors, which are quite expensive. The conventional setting potentiometer requires a separate resistor for each step so that for a large number of setting steps, the cost becomes very high. In addition, a selector switch with a large number of positions becomes unwieldy to operate.

SUMMARY OF THE INVENTION An object of the present invention is to provide a setting device for a control and regulating circuit for the lens current of electron microscopes that has the capability for providing any desired number of setting steps while requiring a minimum number of expensive precision resistors.

It is another object of the present invention to provide a setting device having an output voltage that is quickly and accurately adjustable.

Another object of the invention is to provide a setting device that can be adjusted by one step at a time or by a series of steps in rapid succession.

Another object of the invention is to provide a setting device having a numeric display, preferably independently resettable, for indicating either the absolute value of the function being controlled or the deviation of the function from a preset value.

According to the invention, a setting device for a control and regulating circuit for the electromagnetic coil current of electron microscope lenses comprises a digital counter having an output connected to a digitalto-analog converter for supplying a broad range of incremental control voltages to the control input of the control and regulating circuit in response to stepping pulses applied to the input of the counter. Preferably the digital counter is a forward-backward counter to facilitate either positive or negative adjustment of the lens current.

A pulse generator provides stepping pulses to the input of the digital counter, preferably in response to a manually operated pushbutton control. The generator may supply stepping signals pulse-by-pulse with each operation of the pushbutton, or it may be adjusted to provide either a predetermined number of pulses in series or continuous pulses at an adjustably variable rate for slewing the digital counter rapidly from one setting value to another. If the digital counter is a forward-backward counter, the pulse generator can be switched to supply stepping pulses to either the forward counting input or the backward counting input, depending on whether an increasing or decreasing lens current adjustment is desired. I

The setting device of the invention also preferably includes a numeric display device such as the conventional digitally-actuated displays used in electronic desk-type calculators. The display permits the operator to readily and accurately obtain reproducible values of the coil current.

If the setting device is to be used with the control and regulating circuit for the objective lens in an electron microscope, the display device may preferably have an independent zero reset so that it can be used to indicate the deviation of the lens current from a preset focusing value when making a series of photographic recordings, as described earlier. For greatest operator convenience in such an application, it is desirable to calibrate the numeric display device so that it indicates the defocusing values in length units. This is particularly advantageous when making a series of photographic exposures at a sequence of predetermined different focal planes.

Another desirable feature, if the setting device is to be used to control the focusing current in the objective lens of an electron microscope,is to provide a voltage proportional to the magnification setting of the microscope to the input of a means for adjusting the step width of the digital counter. Alternatively, this magnification voltage is used to control the pulse rate or predetermine the number of pulses in a series of pulses from the pulse generator. This is advantageous because the change in focus value for a given step voltage input to the control and regulating circuit is a direct function of the microscope magnification. The higher the magnification power, the greater is the change in focal length.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a block diagram of a control and regulating circuit for current flow to an electromagnetic lens coil employing the setting device of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, a control and regulating circuit for the lens coil current of an electron microscope is shown to illustrate the intended application of the setting device of the present invention. The control circuit comprises a differential amplifier D, the output of which is connected to the control input 3 of a power amplifier V. In the output circuit of amplifier V, a focusing coil L of an electromagnetic lens is connected in series with a measuring resistor M across terminals 1 and 2 of a current source (not further shown). The differential amplifier D has an input 4 for a variable control voltage and an input 5 for a fixed comparison voltage.

The control voltage supplied to differential anplifier D consists of three components, a course adjustment voltage supplied via a resistor R a fine adjustment voltage supplied via a resistor R and a regulating voltage fed back from measuring resistor M, the three components being summed at input terminal 4.

A voltage divider comprising a fixed resistor R and a variable resistor R is connected to a stabilized reference voltage U the resistance values of R and R being chosen with respect to U to provide a coarse voltage adjustment at a terminal which connects the pickoff arm of R to resistor R Connected to resistor R is a setting device 6 for making fine voltage adjustments in stepped increments. The major components of setting device 6 include a digital counter 7, preferably a forward-backward counter, having outputs 8 to 11 which are connected to a digitallanalog converter 12. The output terminal 13 of digital- /analog converter 12 in turn is connected to resistor R The forward-backward counter 7 has a control input 15 for controlling the counting sequence in a positive direction and a control input R6 for controlling the counting sequence in the negative direction. The two control inputs 15 and 16 are connected to the output of a pulse source 25 for feeding stepping pulses to the forward and backward counting inputs l5 and 16 in response to manual operation of pushbuttons l7 and 18, respectively.

A control knob 20 permits preselection of the number of stepping pulses transmitted by pulse generator 25 for each operation of pushbuttons l7 and 18. When control knob 20 is turned fully clockwise, pulse generator 25 transmits pulses continuously at a rate determined by the setting of control knob 19 so long as pushbutton 17 or 18 is depressed.

in addition to the manual control of pulse rate and pulse number provided by control knobs l9 and 20, respectively, these characteristics may be alternatively controlled automatically by a means 25 responsive to the magnification setting of the microscope.

For many investigations of specimens in electron microscopes it is necessary to accurately reproduce a previously determined focusing current through the objective lens. A numeric display device 21, connected to pulse generator 25 through its own forward-backward digital counter 23 is provided for this purpose. A reset button 22 permits zero reset of the numeric display device 21 independently of the count on forwardbackward counter 7, so that the numeric display can be used to indicate defocusing values from a preset focusing current.

If desired, a setting device (not shown but substantially identical to the one shown in the FIGURE) can be used to provide coarse setting voltages in the same manner as above for fine setting voltages by connecting it to resistor R in place of resistors R and R We claim:

1. A setting device for providing a range of control voltages in incremental steps to the input of a control circuit for regulating the current through an electromagnetic coil of a charged-particle beam apparatus comprising:

a. a digital counter having an input for receiving counting pulses and an output for providing a digital count of the counting pulses supplied to the input and,

b. a digital-to-analog converter having an input connected to the output of the digital counter and an output connected to the input of the control circuit.

2. The setting device of claim 1 wherein the digital counter comprises a forward backward counter.

3. The setting device of claim 1 further comprising:

a pulse generating means connected to the input of the digital counter for supplying counting pulses to the digital counter.

4. The setting device of claim 3 wherein the pulse generating means comprises a manually-operated control means, the pulse generating means supplying a single counting pulse in response to each operation of the control means.

5. The setting device of claim 3 wherein the pulse generating means comprises a manually operated control means, the pulse generating means supplying a series of pulses in response to each operation of the con- I trol means.

6. The setting device of claim 5 wherein the pulse generating means further comprises a rate control means for presetting the repetition rate of the series of pulses supplied by the pulse generating means in response to each operation of the manually-operated control means.

7. The setting device of claim 5 wherein the pulse generating means further comprises a numerical control means for presetting the number of pulses in the series of pulses supplied by the pulse generating means in response to each operation of the manually-operated control means.

8. The setting device of claim 1 further comprising a numeric display device connected to the output of the pulse generating means.

9. The setting device of claim 8 wherein the numeric display device comprises a zero-setting means for setting the count of the display to zero independently of the count of the digital counter.

10. The setting device of claim 9 wherein the electromagnetic coil comprises the objective lens coil of an electron microscope and the numeric display device is calibrated to indicate defocusing values in length units.

1 l. The setting device of claim 6 wherein the electromagnetic coil comprises the objective lens coil of an electron microscope and further comprising a means for adjusting the rate control means as a function of the magnification power of the microscope.

12. The setting device of claim 7 wherein the electromagnetic coil comprises the objective lens coil of an electron microscope and further comprising a means for adjusting the numerical control means as a function of the magnification power of the microscope.

* i i i 

1. A setting device for providing a range of control voltages in incremental steps to the input of a control circuit for regulating the current through an electromagnetic coil of a charged-particle beam apparatus comprising: a. a digital counter having an input for receiving counting pulses and an output for providing a digital count of the counting pulses supplied to the input and, b. a digital-to-analog converter having an input connected to the output of the digital counter and an output connected to the input of the control circuit.
 2. The setting device of claim 1 wherein the digital counter comprises a forward-backward counter.
 3. The setting device of claim 1 further comprising: a pulse generating means connected to the input of the digital counter for supplying counting pulses to the digital counter.
 4. The setting device of claim 3 wherein the pulse generating means comprises a manually-operated control means, the pulse generating means supplying a single counting pulse in response to each operation of the control means.
 5. The setting device of claim 3 wherein the pulse generating means comprises a manually operated control means, the pulse generating means supplying a series of pulses in response to each operation of the control means.
 6. The setting device of claim 5 wherein the pulse generating means further comprises a rate control means for presetting the repetition rate of the series of pulses supplied by the pulse generating means in response to each operation of the manually-operated control means.
 7. The setting device of claim 5 wherein the pulse generating means further comprises a numerical control means for presetting the number of pulses in the series of pulses supplied by the pulse generating means in response to each operation of the manually-operated control means.
 8. The setting device of claim 1 further comprising a numeric display device connected to the output of the pulse generating means.
 9. The setting device of claim 8 wherein the numeric display device comprises a zero-setting means for setting the count of the display to zero independently of the count of the digital counter.
 10. The setting device of claim 9 wherein the electromagnetic coil comprises the objective lens coil of an electron microscope and the numeric display device is calibrated to indicate defocusing values in length units.
 11. The setting device of claim 6 wherein the electromagnetic coil comprises the objective lens coil of an electron microscope and further comprising a means for adjusting the rate control means as a function of the magnification power of tHe microscope.
 12. The setting device of claim 7 wherein the electromagnetic coil comprises the objective lens coil of an electron microscope and further comprising a means for adjusting the numerical control means as a function of the magnification power of the microscope. 